1. Field of the Invention
The present invention relates generally to ultrasound imaging systems and, more particularly, to an ultrasound transducer including a one-twelfth wavelength impedance matching transformer and methods of manufacturing and using the same.
2. Background of the Invention
Intraluminal, intracavity, intravascular, and intracardiac treatment and diagnosis of medical conditions utilizing minimally invasive procedures is an effective tool in many areas of medical practice. These procedures typically are performed using imaging and treatment catheters that are inserted percutaneously into the body and into an accessible vessel, such as the femoral artery, of the vascular system at a site remote from a region of the body to be diagnosed and/or treated. The catheter then is advanced through the vessels of the vascular system to the region of the body to be diagnosed and/or treated, such as a vessel or an organ. The catheter may be equipped with an imaging device, typically an ultrasound imaging device, which is used to locate and diagnose a diseased portion of the body, such as a stenosed region of an artery.
Intravascular imaging systems having ultrasound imaging capabilities generally are known. For example, U.S. Pat. No. 4,951,677, issued to Crowley, the disclosure of which is incorporated herein by reference, describes such an intravascular ultrasound imaging system. An ultrasound imaging system typically contains some type of control system, a drive shaft, and a transducer assembly including an ultrasound transducer. The transducer assembly includes a transducer element and is coupled to the control system by the drive shaft. The drive shaft typically includes an electrical cable, such as coaxial cable, for providing electrical communication between the control system and the ultrasound transducer.
In operation, the drive shaft and the transducer assembly are inserted, usually within a catheter, into a patient""s body and may be positioned near a remote region of interest. To provide diagnostic scans of the remote region of interest within, for example, a coronary blood vessel, the ultrasound transducer may be positioned near or within the remote region of the patient""s body. Diagnostic scans are created when the control system alternately excites and allows sensing by the ultrasound transducer. The control system may direct the ultrasound transducer toward or away from an area of the remote region. When the ultrasound transducer is excited, a transmitting/receiving surface of the transducer element creates pressure waves in the bodily fluids surrounding the ultrasound transducer. The pressure waves then propagate through the fluids within the patent""s body and ultimately reach the region of interest, forming reflected pressure waves. The reflected pressure waves then return through the fluids within the patient""s body to the transmitting/receiving surface of the transducer element, inducing electrical signals within the transducer element. The control system then may collect the induced electrical signals and may reposition the ultrasound transducer to an adjacent area within the remote region of the patient""s body, again exciting and sensing the transducer element. This process may continue until the remote region has been examined sufficiently and a series of induced signals has been collected. The control system then may process the series of induced signals to derive a diagnostic scan and may display a complete image of the diagnostic scan.
To create clear diagnostic scans, it is preferable that the ultrasound transducer have an impedance that is substantially equal to an impedance of the fluids within the patient""s body, thereby maximizing the acoustic coupling between the ultrasound transducer and the patient""s body. The technique of adding one or more one-quarter wavelength impedance matching layers to the transmitting/receiving surface of an ultrasound element generally is known. For example, U.S. Pat. No. 4,523,122, issued to Tone, the disclosure of which is incorporated herein by reference, describes the use of one-quarter wavelength acoustic impedance matching layers on piezoelectric ultrasound transducers. A one-quarter wavelength impedance matching layer provides an impedance transformation between the ultrasound transducer and an operating medium, for example, the fluids within a patient""s body, to allow a better coupling of energy into the operating medium. The impedance transformation depends upon frequency, and the one-quarter wavelength impedance matching layers are used to compensate for the difference between the impedance of the ultrasound transducer and the impedance of the operating medium. The one-quarter wavelength impedance matching layer permits energy to be transmitted between the ultrasound transducer and the operating medium more efficiently, allowing a pressure wave produced by the ultrasound transducer to be introduced into the operating medium with less attenuation. The one-quarter wavelength matching layer also reduces the signal loss experienced when the reflected pressure wave returns from the operating medium and passes into the ultrasound transducer. Therefore, the use of the one-quarter wavelength impedance matching layers may provide for a stronger and sharper pressure wave and, thus, a better image.
However, the production of the one-quarter wavelength impedance matching layer currently poses several problems for ultrasound transducer manufacturers. First, materials with a proper matching impedance can be difficult to procure. Preferably, the impedance of the one-quarter wavelength impedance matching layer is determined substantially at the frequency of the ultrasound wave generated by the ultrasound transducer and is substantially equal to the geometric mean of the impedance of the transducer element and the impedance of the operating medium. Since the geometric mean calculation may result in a non-exact matching of impedances, finding proper materials for the impedance matching layer still may be difficult. Second, ultrasound transducer manufacturers may experience difficulty in uniformly disposing the one-quarter wavelength impedance matching layer on the transmitting/receiving surface of the transducer element. Due to the thickness of the one-quarter wavelength impedance matching layer, ultrasound transducer manufacturers may not be able to avail themselves of modem and more efficient manufacturing techniques, such as thin-film processing and deposition techniques, to produce ultrasound transducers with one-quarter wavelength impedance matching layers. These drawbacks and the limitation on available manufacturing techniques may result in ultrasound transducer manufacturers incurring additional costs that ultimately may be passed on to the patient.
A recent article, xe2x80x9cTry a Twelfth-Wave Transformer,xe2x80x9d by Emerson, the disclosure of which is incorporated herein by reference, describes the use of a one-twelfth wavelength impedance matching transformer for coupling radio equipment to an antenna. The Emerson article suggests that coupling between an antenna cable and an antenna could be achieved by replacing a generally known one-quarter wavelength impedance matching transformer with a one-twelfth wavelength impedance matching transformer. The one-twelfth wavelength impedance matching transformer comprises a first impedance matching cable section and a second impedance matching cable section. The first impedance matching cable section has an impedance substantially equal to an impedance of the antenna and a length of substantially one-twelfth wavelength of a radio signal travelling therein. The second impedance matching cable section has an impedance substantially equal to an impedance of the antenna cable and a length of substantially one-twelfth wavelength of the radio signal passing therethrough. In operation, the first and second impedance matching sections are disposed between and couple the antenna cable and the antenna. The one-twelfth wavelength impedance matching transformer provides the advantages of broad bandwidth, a reduced overall length from one-quarter wavelength to one-sixth wavelength of the radio signal, and ready availability of cable sections to form the one-twelfth wavelength impedance matching transformer.
In view of the foregoing, it is believed that a need exists for an improved ultrasound transducer that overcomes the aforementioned obstacles and deficiencies of currently available ultrasound transducers.
The present invention is directed to an ultrasound transducer incorporating a one-twelfth wavelength impedance matching transformer for minimizing acoustic signal attenuation between a transducer element and an operating medium. The present invention provides the advantages of reduced physical size, enhanced diagnostic images, standardized impedance values, and decreased manufacturing costs.
In one preferred form, an ultrasound transducer in accordance with the present invention may comprise a transducer element for transmitting and receiving ultrasound waves, a first impedance matching layer, and a second impedance matching layer. The transducer element may be capable of generating an ultrasound wave with a preselected wavelength, may have a predetermined impedance, and may include a transmitting/receiving surface that is substantially flat. The transducer element preferably has a thickness substantially equal to one-half wavelength of the ultrasound wave generated thereby. The thickness of the transducer element may be substantially uniform.
The first impedance matching layer may have a thickness substantially equal to one-twelfth wavelength of the ultrasound wave travelling therein and, preferably, has an impedance substantially equal to an impedance of an operating medium, such as water or blood. The second impedance matching layer may have a thickness substantially equal to one-twelfth wavelength of the ultrasound wave passing therethrough and preferably has an impedance substantially equal to the impedance of the transducer element. Preferably, the first impedance matching layer is formed over a transmitting/receiving surface of the transducer element, and the second impedance matching layer is formed over an exterior surface of the first impedance matching layer. Stated somewhat differently, the first impedance matching layer may be deposited on the transmitting/receiving surface of the transducer element, and the second impedance matching layer may be deposited on the exterior surface of the first impedance matching layer.
The ultrasound transducer may also include a pair of electrodes, and the electrodes may be used to couple the ultrasound transducer to an ultrasound imaging system.
In a second preferred embodiment, the transducer element, the first impedance matching layer, and the second impedance matching layer each may be substantially concave in shape for reducing the focal length of the ultrasound transducer.
In a third preferred embodiment, the transducer element, the first impedance matching layer, and the second impedance matching layer each may be substantially convex in shape for increasing the focal length of the ultrasound transducer.
In a fourth preferred embodiment of the ultrasound transducer, the transducer element may include a backing layer. The backing layer may be attached to a back surface of the transducer element to absorb any energy radiating from the back surface of the transducer element. The backing layer preferably is formed from a high impedance material and acts to minimize image distortion resulting from reflections of undesired signals.
It will be appreciated that an ultrasound transducer incorporating a one-twelfth wavelength impedance matching transformer in accordance with the present invention may overcome the manufacturing problems associated with one-quarter wavelength impedance matching layers and enhance the quality of diagnostic images. First, by utilizing a one-twelfth wavelength impedance matching transformer, ultrasound transducer manufacturers may be able to utilize readily available materials. For example, since the first impedance matching layer may be formed from any hydroscopic material having an impedance similar to an impedance of water, ultrasound transducer manufacturers may choose from among a variety of commercial materials to produce the first impedance matching layer. The material for the second impedance matching layer also may be available to ultrasound transducer manufacturers because the second impedance matching layer may be formed from the same piezoelectric material that comprises the transducer element. Second, since each impedance matching layer on the ultrasound transducer has a thickness of only one-twelfth wavelength, more efficient manufacturing techniques, including thin-film processing and deposition techniques, may be used to produce the ultrasound transducer of the present invention. Enhanced diagnostic images are possible due to an increase in signal power transmitted between the ultrasound transducer and the operating medium as well as a reduction in internal reflections in the ultrasound transducer. It also will be appreciated that, by using more efficient manufacturing techniques, substantial savings in manufacturing costs may be passed on to patients required to undergo ultrasound imaging procedures, or to their insurers, through the use of matching transformers in accordance with the present invention.